You can get 1/8 BSPT male to 1/8 NPT female adapters, just do a search. You should just get a manual oil pressure gauge, such as from an auto parts store, they are cheap and more reliable than an electric dash mounted gauge. Once you know you have oil pressure you can figure out whats wrong with the stock gauge, if thats the problem, and fix it. I suggest you don't start it until you know for sure with a gauge that you have oil pressure, you will destroy your brand new engine if it turns out something is wrong.